Y. enterocolitica causes a range of clinical syndromes. In young children and infants the disease manifests as watery diarrhea. In children and young adults infections result in mesenteric lymphadenitis and terminal ileitis. In adults more serious complications can occur such as erythema nodosum and reactive arthritis. Septicemia, with a 50% mortality rate, is a rare but serious complication, occurring most often in individuals whose underlying illnesses result in compromised host defenses. How Yersinia causes these diseases is as yet unknown. However, epithelial cell invasion is thought to be an important aspect of Y enterocolitica pathogenesis. In preliminary experiments we identified two genetic loci, inv and ail, that confer an invasive phenotype on E. coli strain HB101. The long-term goals of our work are to understand the bacteria-host interaction at the molecular level, and to determine how the invasion process is coordinated with other aspects of Yersinia pathogenesis. If we can understand the process of bacterial invasion, it should be possible to devise better methods of treatment and prevention in the future. Specifically we propose to address the following questions: 1) What is the effect on virulence of mutations in inv and/or all? Mutations in inv and ail have been recombined onto the Yersinia chromosome. These mutants will be examined for their virulence and pathogenicity in a mouse model. 2) How does the an gene product promote invasion? Does Ail act directly by interacting with a receptor on the surface of the eukaryotic cell, or indirectly by modifying a bacterial cell surface structure? To address this question the effect on attachment and invasion of antibody against All, and the ability of purified Ail to bind to eukaryotic cells will be examined. Point mutations in All will be isolated and characterized. 3) Are all Ails created equal? Preliminary evidence suggests that Ail from non-American serotypes are less active, which is interesting because these serotypes are less pathogenic. Given the extensive similarity of ail to the Salmonella macrophage survival gene, pagC, the role of ail in the interaction of Y enterocolitica with macrophages will be investigated. 4) How is the expression of the invasion genes regulated? The regulation of expression of the invasion genes and how this is coordinated with expression of other virulence genes will be studied.